Use the Rectangle Diagonals Conjecture to determine if the corners of a room are right angles?

This will probably refer to the fact that in a rectangle the diagonals are equal (i.e. have equal length) and they intersect each other in their middle. Moreover, a rectangle is the only quadrangle having this property. So to check that the floor in a room is a rectangle, you just have to draw the diagonals and check the two properties just mentioned.